The present invention relates to optical devices of the type wherein material layers, or different portions of one layer, having optically distinguishable functions are fixedly supported side by side in accurate geometrical relationship, and in particular to optical devices wherein a plane reflecting surface and a plane diffraction grating surface are held in substantially parallel relationship with respect to each other.
A typical example of a device of this type may be found in some infrared spectrophotometers in the form of a plane mirror and a plane scatter grating, bonded side by side to a generally flat support and rotatable therewith from one to the other of two operative positions. The mirror is normally inserted in the photometric optics, but when the wavelength being scanned is so long that stray radiation of shorter wavelengths begin to affect photometric accuracy, the scatter grating (of suitable grating constant) is substituted. The device may, for example, be located just ahead of the monochromator entry slit.
It is well known that a scatter grating will specularly reflect in the zero order substantially all radiation the wavelength of which is longer than some critical value, which depends on the grating constant, and disperse in the higher orders substantially all radiation having wavelengths shorter than said value. In other words, said grating may be arranged, in a known manner, to act as a plane mirror for certain bands of optical frequencies and as a dispersive element for the higher frequencies. When so arranged, the grating becomes a filter, essentially enabling the wanted frequencies to pass through while rejecting the remainder.
It has become standard practice to mount the plane mirror and this scatter grating separately on a support to form a rotatable assembly capable of being rotated to present to the photometric beam either the plane mirror or the scatter grating, the change over being performed automatically by the operation of machine elements within the spectrophotometer.
Clearly, it is important for the working surface of the mirror and the scatter grating, respectively, to be set in parallel relationship, with a reasonably high degree of accuracy, or the photometric beam will be shifted when switching over from one element to the other. Heretofore, the setting-up operation was difficult and time consuming mainly because of the wavelength of the radiation that must be used for the purpose is the one for which the scatter grating acts as a mirror. Such a wavelength is in a region of the spectrum, which is not only invisible, but also not conveniently detected.